It is well-known that polyethylene terephthalate has gained wide acceptance as a thermoplastic material useful in the manufacture of food packaging products and in particular for one-half gallon and two liter carbonated beverage bottles. In the main, this acceptance has been due to the fact that polyethylene terephthalate, of all the thermoplastic materials available for this application, has the best overall balance of properties such as impact strength, resistance to creep under pressure, low permeability to carbon dioxide and high clarity and gloss.
It is also known that polyethylene terephthalate can undergo degradation during its preparation and conversion into these bottles with the evolution of acetaldehyde, a flavoring agent occurring naturally in many foodstuffs. To a large extent acetaldehyde formation is a result of the thermal scission of beta-hydroxyethyl groups situated on the ends of the polyester chains and to a lesser extent scission of ester linkages to form vinyl ester ends which then can react with the beta-hydroxyethyl end groups to form acetaldehyde. Ultimately, the acetaldehyde becomes entrained in the matrixes of the polyester and the bottles made therefrom and if present in sufficient quantities can adversely affect the taste of the packaged beverage as it diffuses from the bottle into the beverage. Generally, with respect to the one-half gallon and two liter size bottles, the amount of acetaldehyde generated can be maintained at levels insufficient to affect the taste of the packaged beverage through careful control of the polyester and bottle manufacturing processes. However, for smaller sized bottles such as those of one liter capacity or less, control of the manufacturing processes alone cannot achieve the even lower levels of acetaldehyde required for these applications.